An encoder or a transcoder codes images in accordance with various coding systems. The coding systems include MPEG (Moving Picture Experts Group) 2, H.264, and other various systems. In the coding according to these image coding systems, since images are coded by blocks such as macroblocks, there arises variation in the image quality at a block boundary in a coded image. Such a noise is termed a “block noise”.
The block noise is marked especially in a flat portion and a gradation portion of an image. Therefore, in environments where high image quality is required, the noises at block boundaries are reduced by performing a deblocking process.    [Patent Document 1] Japanese Patent Application Laid Open Gazette No. 2008-22404    [Patent Document 2] Japanese Patent Application Laid Open Gazette No. 2008-263529
An encoder disclosed in Patent Document 1 comprises a deblocking filter to reduce the block noises. The encoder of Patent Document 1 performs a deblocking process on images decoded by a local decoder.
In conventional encoders including the encoder of Patent Document 1, a strength evaluation circuit which determines the filter strength of the deblocking filter is incorporated in a coding circuit block. For this reason, acquisition of the image feature value and evaluation of the filter strength are performed simultaneously with coding, and the process therefore becomes complicated and may cause a delay. Further, since the filter strength instantaneously changes in accordance with the image feature value, when a great change of the image feature value is caused by, for example, a scene change or the like, there arises variation in filter strength among pictures and this disadvantageously causes unnatural instability in moving images.
A coding apparatus disclosed in Patent Document 2 uses an average value of filter strengths calculated on a plurality of pictures.